Pozzolanic Reaction
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The pozzolanic activity is a measure for the degree of reaction over time or the
reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
between a
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
and Ca2+ or
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
(Ca(OH)2) in the presence of water. The rate of the
pozzolanic reaction The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic ...
is dependent on the intrinsic characteristics of the pozzolan such as the
specific surface area Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to det ...
, the chemical composition and the active phase content. Physical surface adsorption is not considered as being part of the pozzolanic activity, because no irreversible molecular bonds are formed in the process.


Reaction

The pozzolanic reaction is the
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
that occurs in
portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
upon the addition of
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
s. It is the main reaction involved in the
Roman concrete Roman concrete, also called , is a material that was used in construction in ancient Rome. Roman concrete was based on a hydraulic-setting cement. It is durable due to its incorporation of pozzolanic ash, which prevents cracks from spreading. B ...
invented in
Ancient Rome In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 B ...
and used to build, for example, the
Pantheon Pantheon may refer to: * Pantheon (religion), a set of gods belonging to a particular religion or tradition, and a temple or sacred building Arts and entertainment Comics *Pantheon (Marvel Comics), a fictional organization * ''Pantheon'' (Lone S ...
. The pozzolanic reaction converts a silica-rich precursor with no cementing properties, to a calcium silicate, with good cementing properties. In chemical terms, the pozzolanic reaction occurs between
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
, also known as
portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...
(Ca(OH)2), and
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(written as H4SiO4, or Si(OH)4, in the geochemical notation): :Ca(OH)2 + H4SiO4 → CaH2SiO4·2 H2O or summarized in abbreviated
cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main o ...
: :CH + SH → C-S-H The pozzolanic reaction can also be written in an ancient industrial silicate notations as: : + → or even directly: : + → Both notations still coexist in the literature, depending on the research field considered. However, the more recent geochemical notation in which the Si atom is tetracoordinated by four
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
groups (, also commonly noted ) is more correct than the ancient industrial silicate notation for which
silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
() was represented in the same way as carbonic acid () whose geometrical configuration is trigonal planar. When only considering mass balance, they are equivalent and both are used. The product CaH2SiO4·2 H2O is a
calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds u ...
, also abbreviated as C-S-H in
cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main o ...
, the hyphenation denotes the variable stoichiometry. The atomic (or molar) ratio Ca/Si, CaO/SiO2, or C/S, and the number of water molecules can vary and the above-mentioned
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
may differ. Many
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
s may also contain
aluminate In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic ion, polyatomic anion with a central aluminum atom. Aluminate ...
, or Al(OH)4−, that will react with
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
and water to form calcium aluminate hydrates such as C4AH13, C3AH6 or
hydrogarnet Hydrogrossular is a calcium aluminium garnet series (formula: Ca3Al2(SiO4)3−''x''(OH)4''x'', with hydroxide (OH) partially replacing silica (SiO4)). The endmembers of the hydrogarnet family (grossular, hibschite, and katoite) depend on the deg ...
, or in combination with
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
C2ASH8 or strätlingite (
cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main o ...
). In the presence of anionic groups such as sulfate, carbonate or chloride,
AFm phase An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or , mono, in cement chemist notation. AFm phases are important hydration products in the hydration of hydraulic cements. They are crystall ...
s and AFt or
ettringite Ettringite is a hydrous calcium aluminium sulfate mineral with formula: . It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of ...
phases can form. Pozzolanic reaction is a long term reaction, which involves dissolved silicic acid, water and CaO or Ca(OH)2 or other pozzolans to form a strong cementation matrix. This process is often irreversible. Sufficient amount of free calcium ion and a high pH of 12 and above is needed to initiate and maintain the pozzolanic reaction. This is because at a pH of around 12, the solubility of silicon and aluminium ions is high enough to support the pozzolanic reaction.


Activity determining parameters


Particle properties

Prolonged grinding results in an increased pozzolanic activity by creating a larger specific surface area available for reaction. Moreover, grinding also creates
crystallographic defects A crystallographic defect is an interruption of the regular patterns of arrangement of atoms or molecules in crystalline solids. The positions and orientations of particles, which are repeating at fixed distances determined by the unit cell param ...
at and below the particle surface. The dissolution rate of the strained or partially disconnected silicate moieties is strongly enhanced. Even materials which are commonly not regarded to behave as a
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
, such as
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
, can become reactive once ground below a certain critical particle diameter.


Composition

The overall chemical composition of a
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
is considered as one of the parameters governing long-term performance (e.g. compressive strength) of the blended cement binder, ASTM C618 prescribes that a pozzolan should contain SiO2 + Al2O3 + Fe2O3 ≥ 70 wt.%. In case of a (quasi) one phase material such as blast-furnace slags the overall chemical composition can be considered as meaningful parameter, for multi-phase materials only a correlation between the pozzolanic activity and the chemistry of the active phases can be sought. Many
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
s consist of a heterogeneous mixture of phases of different pozzolanic activity. Obviously, the content in reactive phases is an important property determining the overall reactivity. In general, the pozzolanic activity of phases
thermodynamically stable In chemistry, chemical stability is the thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or in chemical equilibrium with its environment. This may be a dynamic equilibriu ...
at ambient conditions is low when compared to on an equal specific surface basis to less thermodynamically stable phase assemblages. Volcanic ash deposits containing large amounts of volcanic
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
or
zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ï½¥y where is either a metal ion or H+. These pos ...
s are more reactive than
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
sands or
detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephen ...
clay minerals. In this respect, the thermodynamic driving force behind the
pozzolanic reaction The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic ...
serves as a rough indicator of the potential reactivity of a (alumino-)silicate material. Similarly, materials showing structural disorder such as glasses show higher pozzolanic activities than crystalline ordered compounds.


Reaction conditions

The rate of the pozzolanic reaction can also be controlled by external factors such as the mix proportions, the amount of water or space available for the formation and growth of hydration products and the temperature of reaction. Therefore, typical blended cement mix design properties such as the replacement ratio of pozzolan for Portland cement, the water to binder ratio and the curing conditions strongly affect the reactivity of the added pozzolan.


Pozzolanic activity tests


Mechanical tests

Mechanical evaluation of the pozzolanic activity is based upon a comparison of the
compressive strength In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...
of mortar bars containing pozzolans as a partial replacement for
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
to reference mortar bars containing only Portland cement as binder. The mortar bars are prepared, cast, cured and tested following a detailed set of prescriptions.
Compressive strength In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...
testing is carried out at fixed moments, typically 3, 7, and 28 days after mortar preparation. A material is considered pozzolanically active when it contributes to the compressive strength, taking into account the effect of dilution. Most national and international
technical standards A technical standard is an established norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and production methods, ...
or norms include variations of this methodology.


Chemical tests

A
pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
ic material is by definition capable of binding
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
in the presence of water. Therefore, the chemical measurement of this pozzolanic activity represents a way of evaluating pozzolanic materials. This can be done by directly measuring the amount of
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
a pozzolan consumes over time. At high water to binder ratio (suspended solutions), this can be measured by titrimety or by spectroscopic techniques. At lower water to binder ratios (pastes),
thermal analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Several methods are commonly used – these are distinguished from one another by the property which is measured: * D ...
or X-ray powder diffraction techniques are commonly used to determine remaining
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
contents. Other direct methods have been developed that aim to directly measure the degree of reaction of the pozzolan itself. Here, selective dissolutions,
X-ray powder diffraction An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
or scanning electron microscopy
image analysis Image analysis or imagery analysis is the extraction of meaningful information from images; mainly from digital images by means of digital image processing techniques. Image analysis tasks can be as simple as reading bar coded tags or as sophi ...
methods have been used. Indirect methods comprise on the one hand methods that investigate which material properties are responsible for the pozzolan's reactivity with portlandite. Material properties of interest are the (re)active silica and alumina content, the
specific surface area Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass, (with units of m2/kg or m2/g) or solid or bulk volume (units of m2/m3 or m−1). It is a physical value that can be used to det ...
and/or the reactive mineral and amorphous phases of the pozzolanic material. Other methods indirectly determine the extent of the pozzolanic activity by measuring an indicative physical property of the reacting system. Measurements of the
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
, chemical shrinkage of the pastes or the heat evolution by heat flow calorimetry reside in the latter category.


See also

*
Aerated autoclaved concrete Autoclaved aerated concrete (AAC) is a lightweight, precast, foam concrete building material suitable for producing concrete masonry unit like blocks. Composed of quartz sand (SiO2 with impurities), Calcium sulfate, calcined calcium sulfat ...
* Alkali-aggregate reaction * Alkali-carbonate reaction * Alkali-silica reaction *
Calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds u ...
(CSH) *
Calthemite Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4â ...
*
Cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...
*
Cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main o ...
*
Cenosphere A cenosphere is a lightweight, inert, hollow sphere made largely of silica and alumina and filled with air or inert gas, typically produced as a coal combustion byproduct at thermal power plants. The color of cenospheres varies from gray to alm ...
s *
Concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
*
Concrete degradation Concrete degradation may have many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damages are caused by the ...
*
Energetically modified cement Energetically modified cements (EMCs) are a class of cements made from pozzolans (e.g. fly ash, volcanic ash, pozzolana), silica sand, blast furnace slag, or Portland cement (or blends of these ingredients). The term "energetically modified" ar ...
(EMC) *
Fly ash Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...
*
Geopolymer Geopolymers are inorganic, typically ceramic, alumino-silicate forming long-range, covalently bonded, non-crystalline (amorphous) networks. Obsidian (volcanic glass) fragments are a component of some geopolymer blends. Commercially produced geopoly ...
*
Metakaolin Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Minerals that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than ce ...
*
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th c ...
*
Pozzolan Pozzolans are a broad class of Silicon_dioxide, siliceous and aluminium oxide, aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically ...
*
Pozzolana Pozzolana or pozzuolana ( , ), also known as pozzolanic ash ( la, pulvis puteolanus), is a natural siliceous or siliceous- aluminous material which reacts with calcium hydroxide in the presence of water at room temperature (cf. pozzolanic reactio ...
*
Rice husk ash Rice hulls (or rice husks) are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel. Rice hulls are p ...
*
Roman concrete Roman concrete, also called , is a material that was used in construction in ancient Rome. Roman concrete was based on a hydraulic-setting cement. It is durable due to its incorporation of pozzolanic ash, which prevents cracks from spreading. B ...
*
Silica fume Silica fume, also known as microsilica, (CAS number 69012-64-2, EINECS number 273-761-1) is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon a ...
*
Sodium silicate Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...


References


Further reading

* Cook D.J. (1986) Natural pozzolanas. In: Swamy R.N., Editor (1986) ''Cement Replacement Materials'', Surrey University Press, p. 200. * Lechtman H. and Hobbs L. (1986) "Roman Concrete and the Roman Architectural Revolution", ''Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future'', edited by W.D. Kingery and published by the American Ceramics Society, 1986; and Vitruvius, Book II:v,1; Book V:xii2. * McCann A.M. (1994) "The Roman Port of Cosa" (273 BC), ''Scientific American, Ancient Cities'', pp. 92–99, by Anna Marguerite McCann. Covers, ''hydraulic concrete, of "Pozzolana mortar"'' and the ''5 piers, of the
Cosa Cosa was a Latin colony founded in southwestern Tuscany in 273 BC, on land confiscated from the Etruscans, to solidify the control of the Romans and offer the Republic a protected port. The Etruscan site (called ''Cusi'' or ''Cosia'') may have b ...
harbor, the Lighthouse on pier 5,'' diagrams, and photographs. Height of Port city: 100 BC. * {{Cite journal , last = Mertens , first = G. , author2=R. Snellings , author3=K. Van Balen , author4=B. Bicer-Simsir , author5=P. Verlooy , author6=J. Elsen , year = 2009 , title = Pozzolanic reactions of common natural zeolites with lime and parameters affecting their reactivity , journal = Cement and Concrete Research , volume = 39 , issue = 3 , pages = 233–240 , doi = 10.1016/j.cemconres.2008.11.008 , issn = 0008-8846 Cement Concrete Masonry